1. Field of the Invention
The present invention relates to a method of growing a nitride crystal and more particularly, to a method of growing a nitride crystal of a group III element or elements such as gallium (Ga), aluminum (Al), indium (In), and boron (B) which is one of III-V compound semiconductor materials and applicable to light-emitting, cladding, or conductive layers of semiconductor light-emitting elements or devices.
2. Description of the Related Art
In recent years, crystalline nitrides of group III elements such as gallium nitride (GaN) have been attracting a great deal of attention as materials for light-emitting semiconductor elements or devices that emit blue light and high heat-resistant transistors. It is known that GaN bulk crystal is difficult to be grown due to high dissociation pressure of nitrogen (N) and thus, it has been usually produced by using a variety of epitaxial growth methods, such as Metal-Organic Vapor Phase Epitaxy (MOVPE) and Molecular Beam Epitaxy (ME).
In the MOVPE method, typically, trymethylgallium (TMG), trymethylaluminum (TMA), and/or trymethylindium (TMI) is/are used as the group III component material and gaseous ammonia (NH.sub.3) is used as the group V component material. TMG, TMA, and/or TMI is/are reacted with NH.sub.3 at high temperature to thereby grow epitaxially nitride crystal of Ga, Al, and/or In on a crystalline substrate. The MOVPE method has been forming a main stream in crystal growth methods of producing III-V compound semiconductor materials for light-emitting semiconductor elements or devices such as Light-Emitting Diodes (LEDs) and laser diodes (LDs), because the method generates nitride crystal with desired quality.
Double heterojunction is essential for semiconductor diode lasers, because the active or light-emitting layer needs to be sandwiched by a pair of cladding layers with a lower refractive index than that of the active layer, thereby controlling the mode of light to be emitted. With semiconductor diode lasers using nitride semiconductor materials, typically, the active layer includes a quantum well (QW) structure formed by InGaN sublayers and the pair of cladding layers are made of AlGaN. Moreover, if semiconductor diode lasers are of the current-injection type, they further requires p- or n-type conductive nitride layers serving as current-injection layers and electrode layers in addition to the active layer and the pair of cladding layers. Accordingly, to enhance the light-emitting performance or characteristics of laser diodes, the key is how to grow higher-quality semiconductor layers applicable to the active, cladding, and conductive layers with as good controllability as possible.
In the growing process of AlGaN layers applicable to the cladding layers of laser diodes, it has been known that TMA and TMG as the group III component material react with NH.sub.3 as the group V component material. In other words, TMA and TMG cause an undesired intermediate or parasitic reaction with NH.sub.3 during the growth process of AlGaN, which was reported by C. H. Chen et al. in Journal of Electronic Materials, Vol. 25, No. 6, 1996, pp. 1004-1008. Thus, high-quality AlGaN layers are difficult to be formed with good reproducibility. This intermediate or parasitic reaction can be understood as a reaction that TMA reacts with NH.sub.3 to produce stable adducts and then, these adducts cause a polymerization reaction with TMA and TMG. This understanding was disclosed by Matsumoto et al. in the Technical Report of the Institute of Electronics, Information, and Communications Engineers (IEICE), Vol. 98, No. 384, pp. 45-51.
The intermediate reaction among TMA and TMG and NH.sub.3 generates some reaction products with extremely low vapor pressure and therefore, the reaction products tend to be adhered onto the inner walls of the transport or supply tubes or pipes to the substrate on which the AlGaN layers are grown. Thus, there arises a problem that the growth rate of AlGaN is extremely low. Furthermore, because the intermediate reaction is sensitive to the partial pressure of the materials (i.e., TMA and TMG and NH.sub.3) and the ambient temperature, there arises another problem that the Al composition of AlGaN crystal varies widely in the growth processes and that the Al composition of AlGaN crystal fluctuates within the substrate or wafer.
The wide variation and fluctuation of the Al composition of AlGaN induces serious problems. For example, the refractive index distribution is disordered in the laser diodes and as a result, the performance such as the threshold current density of laser oscillation degrades. Thus, it is an extremely important problem to suppress the intermediate reaction.
Nitride semiconductor materials of this sort have another problem that conductivity control for p-type GaN and p-type AlGaN crystals is difficult. To produce p-type semiconductor nitride crystal, typically, magnesium (Mg) is doped as a p-type dopant or acceptor. In this case, however, a p-type nitride crystal thus grown is of high electrical resistance and does not exhibit any p-type conductivity. It has been understood that the cause of this phenomenon is that Mg atoms as the acceptor are passivated by hydrogen (H) atoms originated from NH.sub.3 as the group V material and/or gaseous hydrogen (H.sub.2) as the carrier gas, which was reported by J. A. Van Vechten et al., in the Japanese Journal of Applied Physics, Vol. 31, 1992, pp. 3662-3663.
To prevent the passivation of Mg atoms due to the H atoms, electron-beam irradiation (which was disclosed by H. Amano et al., in the Japanese Journal of Applied Physics Vol. 28, No. 12, 1989, pp. L 2112-L2114) or heat treatment in an inert atmosphere (which was disclosed by S. Nakamura et al., in the Japanese Journal of Applied Physics Vol. 35, 1996, pp. L74-L76) can be performed to reactivate the Mg atoms. The reactivation process by the heat treatment is simple and convenient; however, it has a problem that the surface of a nitride semiconductor layer is damaged by applied heat to thereby increase the contact resistance and that the count of the necessary process steps is increased.
On the other hand, to suppress the passivation of Mg atoms as the acceptor by a carrier gas, gaseous nitrogen (N.sub.2) may be substantially used as a carrier gas, which was disclosed in the Japanese Non-Examined Patent Publication No. 10-135575 published in May 1998. In this case, however, if the amount of H.sub.2 in the carrier gas is set at zero, there arises a problem that the crystallinity of a nitride semiconductor layer deteriorates. This is due to the fact that H.sub.2 in the carrier gas serves as a reducing agent and therefore, it reduces hydrocarbon radicals generated by decomposition of oxygen (O.sub.2) in the atmosphere and of organometallic compounds. Moreover, there arises another problem that the effect of the activated H atoms generated by decomposition of NH.sub.3 cannot be eliminated and thus, the passivation of Mg atoms due to the H atoms is unable to be suppressed effectively.